editor's blog
Subscribe Now

From Relative to Absolute Altitude

GPS is notoriously inaccurate when it comes to vertical positioning. And it disappears entirely inside buildings. So pressure sensors are used to help calculate vertical positioning.

The thing is, a pressure sensor decides your altitude based on the pressure of the air, so it must be comparing it to some baseline. The problem with that is that there is no firm baseline pressure: weather, as we all know, affects the air pressure.

That means that pressure is, first of all, a moving target. Secondly, we can never really know our absolute altitude, only relative.

I posed these questions in a conversation with the Bosch Sensortec team at the MEMS Executive Congress where they were discussing the upcoming release of their new pressure sensors. They talk about being able to handle absolute altitude, so the obvious question is, what about the weather?

There are two pieces to the answer. The first deals with the fact that the baseline pressure isn’t constant. However, compared to pressure changes due to typical motion, the weather pressure changes extremely slowly. (If it’s changing so fast that it could be confused with you moving around, then navigation error is the least of your problems.) From a signal standpoint, the pressure changes of interest can be extracted with a high-pass filter, at least conceptually. More simply, you can think of it as a differential-mode measurement, with actual weather pressure being a common-mode error that’s subtracted out.

That allows you to get a reasonably accurate measure of relative altitude, but what about absolute altitude? Now you need to compare yourself to a sea-level baseline, and that baseline does depend on the weather. Well, there’s no magic available on this. The Bosch Sensortec software can get the data necessary to correct for the current sea-level pressure from the internet. Given that external sanity check, a pressure sensor can provide absolute altitude.

There are a couple other “faster-twitch” effects that can confuse pressure interpretation. The first is simply the fact that some buildings or rooms may have higher or lower air pressure based on the air conditioning or intentional implementation of things like positive pressure for a clean room. Even just opening a door can send a pressure surge. These effects won’t be eliminated or “de-convoluted” in the same way that weather impacts can be. Instead, the pressure data must be fused with other data to decide whether the pressure change reflects a change in altitude. Specifically, if an inertial sensor shows no vertical motion, then the pressure change can be “ignored” (although now it becomes the new baseline).

Pressure measurements also depend on temperature: a local temperature change can register as a pressure change when in fact the pressure didn’t change. Good temperature compensation is required (which is essentially data fusion between a thermometer and a pressure sensor); a pressure sensor less affected by temperature (as is claimed by Bosch Sensortec for their new BMP280) can also help.

Leave a Reply

featured blogs
Jul 3, 2020
[From the last episode: We looked at CNNs for vision as well as other neural networks for other applications.] We'€™re going to take a quick detour into math today. For those of you that have done advanced math, this may be a review, or it might even seem to be talking down...
Jul 2, 2020
Using the bitwise operators in general, and employing them to perform masking operations in particular, can be extremely efficacious....
Jul 2, 2020
In June, we continued to upgrade several key pieces of content across the website, including more interactive product explorers on several pages and a homepage refresh. We also made a significant update to our product pages which allows logged-in users to see customer-specifi...

Featured Video

Product Update: DesignWare® Foundation IP

Sponsored by Synopsys

Join Prasad Saggurti for an update on Synopsys’ DesignWare Foundation IP, including the world’s fastest TCAMs, widest-voltage GPIOs, I2C & I3C IOs, and LVDS IOs. Synopsys Foundation IP is silicon-proven in 7nm in more than 500,000 customer wafers, and 5nm is in development.

Click here for more information about DesignWare Foundation IP: Embedded Memories, Logic Libraries & GPIO

Featured Paper

Cryptography: How It Helps in Our Digital World

Sponsored by Maxim Integrated

Gain a basic understanding of how cryptography works and how cryptography can help you protect your designs from security threats.

Click here to download the whitepaper

Featured Chalk Talk

Improving Battery-Life with Ultra Low-Power Processors

Sponsored by Mouser Electronics and NXP

Battery life is critical in today’s mobile device designs, and designing-in ever-larger batteries causes all sorts of awkward compromises. The best strategy is to lower power consumption, and the processor is a great place to start. In this episode of Chalk Talk, Amelia Dalton chats with Nik Jedrzejewski of NXP about the new NXP 7ULP, and how it will help you cut power consumption in your mobile design.

Click here for more information about NXP Semiconductors i.MX 8M Mini Applications Processors